(1) Field of the Invention
Present invention relates to method and device for measuring half-wave voltage (AC half-wave voltage) of a Mach-Zehnder type optical modulator (hereinafter refer to as MZ type optical modulator), especially a method and a device for measuring the half-wave voltage at high frequency of a MZ type optical modulator, which corresponds to high frequency modulation used in high-speed and high-capacity optical fiber communications.
(2) Related Art Statement
An optical modulator is the primary element, which is used in transmitting parts of optical communications, especially, in recent high-speed and high-capacity optical communication, MZ type optical modulator, which is made by using LiNbO3 (LN) is used in huge range with its features such as high-speed, wide wavelength band, and low-chirp.
As shown in FIG. 1, MZ type optical modulator 1 is composed of light guide 2 which is for guiding light waves, on the substrate that has an electro-optical effect, and the electrodes (not shown) which is for applying high-speed modulation signal of micro wave band to said light waves, and so on. The principle of MZ type optical modulator's operation is that the input light from one end of the light guide 2 is divided on the way and because the lights pass inside the substrate of which the refractive index has changed dependent on the amount of electronic signal voltage which applied from signal source, speed difference occurs between mutual divided lights, and as the two divided lights converge, phase difference occurs, and the combined output light shows an intensity change which respond to said electronic signal.
The FIG. 2 shows the change of output light (I) in relation to the change of input voltage (V) of signal source 3 which is applied to MZ type optical modulator 1, and generally, as the input voltage V increases, light output I draw a curve that vibrates as a sine wave in certain range. As in FIG. 2, an input voltage range between minimum point and maximum point of the light output is called half-wave voltage Vπ, and when ON/OFF switching control using optical modulator in optical communication is handled, the value Vπ is important to determine the voltage of electronic signal which is applied to optical modulator.
Even when the same optical modulator is used, half-wave voltage changes according to the electronic signal frequency which is applied to optical modulator, and also, as optical communication become high-speed and high-capacity recently, drive frequency of optical modulator has also become high, so it is required to measure accurate half-wave voltage at 10 GHz or higher frequency.
To measure half-wave voltage, there are several methods such as FIG. 3 which shows the way to observe the light output directly (prior example 1), or as the FIG. 5 shown in U.S. Pat. No. 6,204,954 which is a method using the average power of output light (prior example 2).
In the prior example 1, as shown in FIG. 3, light from laser source 4 enters to MZ type optical modulator 1, and at the same time, the voltage which superimposed bias voltage from bias DC power 32 to high-frequency AC signal 31 by using bias T 33 is applied to MZ type optical modulator 1. Then the light from MZ type optical modulator 1 is detected by using high-speed photo detector 51, and by observing the detected power on sampling oscilloscope 52, half-wave voltage is measured.
The method for measuring by prior example 1 is that when the relation between input voltage V and output light I (V-I characteristic) of MZ type optical modulator 1 is like graph A in FIG. 4, if bias voltage VB is superimposed to high-frequency AC signal which has peak—peak voltage amplitude value Vp-p and input voltage as graph B is applied to MZ type optical modulator 1, output light becomes like graph C, and the wave form of graph C is observed directly on sampling oscilloscope 52.
As carrying this direct observation, by adjusting the voltage value of Vp-p and VB to make peak—peak amplitude of graph C maximum and measuring Vp-p at its peak, half-wave voltage Vπ of MZ type optical modulator 1 will be determined (Vπ=Vp-p).
However, with the method of measuring by prior example 1, it is hard to observe an accurate light waveform (graph C in FIG. 4) at high-frequency over 10 GHz, because of the problem with frequency characteristics of light receiving system such as high-speed photo detector 51. Therefore, it cannot measure half-wave voltage precisely.
On the other hand, in the prior example 2, as shown in FIG. 5, the voltage of high-frequency AC signal 31 superposed with bias voltage from DC power 32 by bias T 33, is applied to the MZ type optical modulator 1 which the laser is passing through. Then it is comprised to diverge the light from MZ type optical modulator 1 at light coupler 53, to detect one light by light power meter 54 and another light by photo detector 55, and to introduce the detected power of the photo detector to spectrum analyzer 56.
The method of measuring by the prior example 2 is that when relationship between input voltage V and light output I (V-I characteristics) of MZ type optical modulator 1 shows the pattern like graph A in FIG. 6, after adjusting the bias voltage VB to the input voltage showing peak volume in graph A of V-I characteristic (bias point adjustment), it is carried out to measure average power volume of light output (graph C1) in the case (graph B1) of superposing high-frequency AC signal which has peak—peak voltage amplitude volume Vp-p to bias voltage VB, light output volume (graph C2) in not adding high-frequency AC signal (only bias voltage VB, graph B2), and Vp-p of high-frequency AC signal. Then by using that the V-I characteristic becomes to sine function, it is possible to find out half-wave voltage Vπ of MZ type optical modulator.
Handling with the method of measuring by the prior example 2, it is able to measure an accurate half-wave voltage at high-frequency, but the bias point adjustment for MZ type optical modulator is necessary and there is a problem that an accurate half-wave voltage measurement is difficult when there is a fluctuation in output light because of the bias point variation. Also, there are many parameters to use for half-wave voltage calculation so the calculation is becoming cumbersome and complicated.
Present invention solves these problems and offers a method and device for measuring that is able to measure half-wave voltage of MZ type optical modulator accurately at high-frequency, does not depend on the bias point variation of the optical modulator, and in which the parameters for calculation are simple.